1. Field of the Invention
This invention relates to a secondary air supply control system for an internal combustion engine, which controls the supply of secondary air to an upstream side of a catalyst arranged in an exhaust pipe of the engine to activate the catalyst, thereby enhancing exhaust emission characteristics.
2. Description of the Prior Art
In general, a catalyst, such as a three-way catalyst, arranged in an exhaust pipe of an internal combustion engine is not active enough for purification of exhaust gases until it is heated up to a predetermined temperature. To cope with this inconvenience, some conventional internal combustion engines are provided with a secondary air supply control system for supplying secondary air to the upstream side of the catalyst at the start of the engine by operating an electromagnetic valve and an electric air pump to burn unburned combustible components in exhaust gases, such as CO and HC, thereby activating (warming up) the catalyst. A secondary air supply control system of this kind was proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 6-129241. In this secondary air supply control system, an engine coolant temperature THWST at the start of the engine is sensed by an engine coolant temperature sensor, then a supply time period CDANKIX is calculated based on the sensed engine coolant temperature THWST, and secondary air is supplied to the upstream side of the catalyst over the calculated supply time period CDANKIX to thereby control activation of the catalyst. That is, the secondary air supply control system uses the engine coolant temperature THWST sensed at the start of the engine as a parameter reflecting a temperature of the catalyst at the start of the engine to determine the supply time period CDANKIX over which secondary air is to be supplied. More specifically, as the engine coolant temperature THWST is lower, the supply time period CDANKIX is set to have a larger value.
However, since the above secondary air supply control system uses the engine coolant temperature THWST as the parameter reflecting the catalyst temperature, secondary air can be supplied excessively or insufficiently. This results from the fact that the engine coolant temperature THWST sensed when the engine is restarted cannot always reflect the catalyst temperature accurately because the catalyst temperature at the restart of the engine depends on an operating condition of the engine before stoppage, a stop time period between the stoppage and the restart of the engine, and so forth, even if the engine coolant temperature THWST is identical. The excessive supply of secondary air can cause overheating of the catalyst and generate untoward noises. In addition, an excessively long operation time period over which the electromagnetic valve and the electric air pump are operated for supplying the secondary not only shortens the service life of the electromagnetic valve and that of the electric air pump but also degrade fuel economy and fast consumption of a battery. On the other hand, the insufficient supply of secondary air causes degradation of exhaust emission characteristics due to insufficient activation of the catalyst.